Conventionally, a vehicular power line communication device (PLC system, PLC: Power Line Communications) is known (e.g., JP 2006-67421A). In this system, a camera that is mounted in a vehicle such as a car to photograph its periphery, and a receiving device that receives a video signal from the camera to perform image processing and the like are connected to enable data communications with each other through an electric power line wired in the vehicle and transmit the video signal from the camera to the receiving device through the power line.
In this sort of PLC system, to achieve both high noise resistance and high speed data transmission, the orthogonal frequency division multiplexing (OFDM) method is generally used as a communication method (e.g., JP 2007-49531A).
In the PLC system that performs data communications by the OFDM method, some of sub-carriers allocated for communications are prohibited from use to avoid interference with other communication services using the same frequency band and normally perform data communications without undergoing the influence of noises.
Therefore, when the PLC system is formed in a vehicle, it is proposed to employ the OFDM method as a communication method for detecting the frequencies and levels of noises superimposed on a power line to enable normal data communications without undergoing the influence of the noises and switch sub-carriers used for communications according to the detection result.
However, the vehicle includes an engine and various noise generating sources (various actuators such as a motor, electromagnetic valve, and solenoid). Since the frequencies and levels of noises superimposed on a power line change frequently according to the operating states of parts that generate the noises, it is difficult to detect all noises superimposed on the power line and set a sub-carrier to be used for communications.
Moreover, since the vehicle includes numerous noise generating sources, such wide band pulse-related noises as to cover a frequency band of all sub-carriers usable in communications of the OFDM method may occur. Such wide band noises are caused to be mixed with all sub-carriers in communications of the OFDM method, and bring about the inexistence of sub-carriers usable for the communications, with the result that the communications are stopped as long as the wide band noises occur.
On the other hand, to prevent such a drawback, if the PLC system is formed in a vehicle, it is proposed to use the spread spectrum method (SS method) having higher noise resistance than the OFDM method. However, with the SS method, unlike the OFDM method, high speed communications cannot be achieved, and the amount of data transmittable and receivable per unit time becomes smaller.